This invention relates to an electromagnetic actuator for a valve stem having a stator and a rotor separated from each other by a nonmagnetic tubular member, and adapted for driving an actuator such as a valve which is to be operated in the atmosphere of a high-temperature or corrosive fluid.
The known motor for actuating a valve is partially disclosed, for instance, in published U.S. Pat. Nos. 4,452,423 and 4,789,132 in which a stator producing a rotating magnetic field around the wall the tubular member is provided outside the tubular member, while a rotor rotatably supported by bearings is provided inside the tubular member at a position opposing the stator. The rotating movement of the rotor is converted into a linear movement of a valve member by a screw mechanism so as to control an opening and closing operation of the valve. The stator, in a type of the known motor for actuating a valve, is formed as a magnetic collar including a plurality of permanent magnets freely rotatable around the outside of the tubular member, while in another type it is constructed to include ordinary stator coils; the rotor in these types of devices is made of permanent magnets or a ferromagnetic member capable of producing eddy current (U.S. Pat. Nos. 4,452,423 and 4,789,132).
However, in a case where the stator is constructed in the form of a magnetic collar, a mechanism slidably rotating the magnetic collar around the outside of the tubular member is required. On the other hand, where the stator is constructed to include ordinary stator coils, the diameter of the stator becomes excessively large when the size of the coils is increased to increase the magnetomotive force. Furthermore, the number of magnetic poles is restricted by the shape and size of stator coils, so that the precision adjustment of the rotating angle is made difficult.
In addition, in a case where the rotor is made of permanent magnets, the operating property of the permanent magnets tends to be deteriorated by the high-temperature fluid, and the joining strength between the permanent magnets and a member supporting the same will also be reduced so as to increase the possibility of the permanent magnets being dropped out of the supporting member.